Alloy solder and alloy solder manufacturing method

ABSTRACT

The present invention solves the problem of the bonding strength of a Pb-free alloy solder being inferior to that of the conventional Pb-containing alloy solder and provides a Pb-free alloy solder satisfactory in bonding reliability. 
     An alloy solder is manufactured by adding a predetermined amount of carbon to a Pb-free solder in a high-temperature atmosphere of a temperature in the range of 800° C. to 1200° C. An alloy solder manufacturing method includes a melting process for melting a Pb-free solder by heating the Pb-free solder in a high-temperature atmosphere of a temperature in the range of 800° C. to 1200° C., a carburizing process for carburizing the molten Pb-free solder held in the high-temperature atmosphere by adding a predetermined amount of carbon to the molten Pb-free solder, a stirring process for stirring a mixture of the molten Pb-free solder and carbon, and a cooling process for cooling the mixture of the Pb-free solder and carbon stirred by the stirring process and poured into a mold to solidify the mixture.

TECHNICAL FIELD

The present invention relates to an alloy solder and, more specifically, an alloy solder produced by carburizing a solder.

BACKGROUND ART

As is generally known, a Pb—Sn alloy solder is a representative alloy solder. However, restrictions have been placed on the use of the solder to avoid exerting bad influence on the environment because lead contained in the alloy solder has a poisonous effect.

Therefore, there has been a demand for a Pb-free alloy solder not containing harmful lead. Various Pb-free alloy solders are proposed in Patent documents 1 to 5. A representative on of those Pb-free alloy solders is a Sn—Ag alloy solder containing Sn and 3.5% Ag. This Sn—Ag alloy solder having a comparative low melting point of 221° C. is used widely as a Pb-free alloy solder at the present.

However, the Sn—Ag alloy solder is inferior to the conventional alloy solder containing lead in bonding strength. Therefore, from the viewpoint of safety, inhibition of the use of alloy solders containing lead in fields in which high bonding reliability is essential, such as the fields of vehicles, has been postponed and Pb-containing alloy solders are used in those fields. However, in consideration of environmental effect, Pb-containing solders should be replaced with solders not containing Pb as soon as possible in all those fields. Therefore, it is desired to develop a Pb-free solder that is no way inferior to the Pb-containing solder in bonding reliability.

Patent document 1: JP 2007-237252 A

Patent document 2: JP 2006-255784 A

Patent document 3: JP 2002-346788 A

Patent document 4: JP 2001-225188 A

Patent document 5: JP H10-6075 A

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

It is an object of the present invention to improve the bonding reliability of a Pb-free solder to solve the problem of the bonding strength of a Pb-free alloy solder being inferior to that of the conventional Pb-containing alloy solder.

Means for Solving the Problem

Usually, the conventional Pb-containing alloy solder is manufactured in a low-temperature atmosphere of a temperature between 250° C. and 400° C. The present invention is based on the inventor's knowledge that made possible the addition of carbon and the practically acceptably uniform distribution of carbon in a high-temperature atmosphere of a temperature higher by far than that between 250° C. and 450° C.

To achieve the object, the present invention provides an alloy solder manufactured by adding carbon to a Pb-free solder in a predetermined carbon content in a high-temperature atmosphere.

The temperature of the high-temperature atmosphere for manufacturing the alloy solder is in the range of 800° C. to 1200° C.

The predetermined carbon content is in the range of 0.01 to 0.7 wt %.

The carbon is graphite of a hexagonal system.

The Pb-free solder contains 96.5 wt % Sn, 3 wt % Ag and 0.5 wt % Cu.

The Pb-free solder contains 99.3 Wt % Sn and 0.7 wt %

Cu.

The Pb-free solder contains 99.0 wt % Sn, 0.7 wt % Cu and 0.3 wt % Ag.

The present invention provides an alloy solder manufacturing method including:

a melting process for melting a Pb-free solder by heating a high-temperature metal melting system containing the Pb-free solder at a temperature for a high-temperature atmosphere;

a carburizing process for carburizing the molten Pb-free solder held in the high-temperature atmosphere by adding carbon as a recarburizer to the molten Pb-free solder such that the Pb-free solder has a predetermined carbon content;

a stirring process for stirring a mixture of the molten Pb-free solder and carbon; and

a cooling process for cooling the mixture of the Pb-free solder and carbon stirred by the stirring process for stirring the mixture of the Pb-free solder and carbon and poured into a mold to solidify the mixture.

In the alloy solder manufacturing method, the temperature of the high-temperature atmosphere is in the range of 800° C. to 1200° C.

The predetermined carbon content is in the range of 0.01 to 0.7 wt %.

In the alloy solder manufacturing method, the high-temperature metal melting system includes a melting furnace to be charged with the Pb-free solder and the recarburizer, a sealed heating space forming part for forming a sealed heating space above the melting furnace, a heating unit for supplying a heating fuel into the sealed heating space to heat the sealed heating space and the furnace, and an exhaust pipe connected to the heating space forming part.

In the alloy solder manufacturing method, the supply of the heating fuel is regulated such that a gas exhausted through the exhaust pipe during the melting process contains no oxygen.

The alloy solder manufacturing method further includes a carbon content adjusting process for adjusting the carbon content of the mixture to a desired carbon content by charging a low-temperature melting system with the mixture solidified by the cooling process, melting the mixture in a low-temperature atmosphere and charging an additional amount of the Pb-free solder into the low-temperature melting system, and a recooling process for cooling and solidifying the mixture having the predetermined carbon content adjusted by the carbon content adjusting process by pouring the molten mixture into a mold.

The alloy solder manufacturing method further includes an analyzing process for determining the carbon content of the mixture solidified by the cooling process to be executed before the carbon content adjusting process; wherein the carbon content adjusting process determines the predetermined additional amount on the basis of the result of the analysis made by the analyzing process.

In the alloy solder manufacturing method, the mixture solidified by the cooling process has a higher one of carbon contents in the range of 0.01 to 0.7 wt %, and the desired carbon content is a lower one of the carbon content in the range of 0.01 to 0.7 wt %.

In the alloy solder manufacturing method, the temperature of the low-temperature atmosphere is in the range of 250° C. to 400° C.

In the alloy solder manufacturing method, the carbon is graphite of a hexagonal system.

An alloy solder of the present invention is manufactured by the alloy solder manufacturing method.

The alloy solder of the present invention is a Pb-free alloy solder harmless to the environment and capable of solving the problem of the bonding strength of a Pb-free alloy solder being inferior to that of the conventional Pb-containing alloy solder and of improving bonding reliability by remarkably improving bonding strength.

The alloy solder manufacturing method of the present invention can easily and effectively manufacturing a Pb-free alloy solder ensuring remarkably improved bonding strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image of the interface between a Cu substrate and a specimen A containing Sn, 3.5 wt % Ag and 0.03 wt % C and bonded to the Cu substrate formed by a SEM;

FIG. 2 is an image of the interface between a Cu substrate and a specimen B containing Sn and 3.5 wt % Ag and bonded to the Cu substrate formed by a SEM;

FIG. 3 is a schematic view of a high-temperature metal melting system; and

FIG. 4 is a schematic view of a low-temperature metal melting system.

BEST MODE FOR CARRYING OUT THE INVENTION

A principal feature of the present invention is to provide an alloy solder by adding carbon to a Pb-free solder in a high-temperature atmosphere. Preferred embodiments of the present invention will be described. The present invention is not limited to the embodiments specifically described herein.

An alloy solder of the present invention is manufactured by adding carbon to a Pb-free solder in a predetermined carbon content in a high-temperature atmosphere. The effect of the high-temperature atmosphere is as follows. Whereas most conventional Pb-free solders are manufactured in a low-temperature atmosphere in the range of 250° C. to 400° C., the present invention adds carbon to a Pb-free solder in the high-temperature atmosphere in a high temperature range of, for example, 800° C. to 1200° C. higher by far than the low-temperature atmosphere in the range of 250° C. to 400° C. in which the conventional Pb-free alloy solder is manufactured to distribute carbon in a practically acceptably uniform distribution.

Desirably, the temperature of the high-temperature atmosphere of the present invention is in the range of 800° C. to 120° C. When carbon is added to the Pb-free solder in a low-temperature atmosphere below 800° C., carbon masses cannot be broken into particles and hence an effective alloy solder cannot be manufactured. In a high-temperature atmosphere of a temperature above 1200° C., the solder tends to boil in the high-temperature metal melting system. Therefore, such a high-temperature atmosphere is unsuitable for practical solder manufacture. Although carbon needs to be added to the solder in a high-temperature atmosphere of a higher temperature, carbon can be mixed in the solder in an ideal form at a temperature not higher than 1200° C. Addition of carbon to the solder in a high-temperature atmosphere of a temperature above 1200° C. increases fuel cost and is uneconomical and wasteful.

Preferably, carbon is added to a Pb-free solder such that the carbon content of an alloy solder of the present invention has a carbon content in the range of 0.01 to 0.7 wt %. The carbon contained in the alloy solder, in itself, does not directly contribute to the bonding strength of the alloy solder. Therefore, there is a proper carbon content for the alloy solder. If the carbon content is above 0.7 wt %, it is possible that the bonding strength of the alloy solder diminishes. If the carbon content is below 0.01 wt %, the carbon is not effective and a necessary bonding strength cannot be achieved.

It is considered that an alloy solder having a higher carbon content has higher strength and higher hardness. The respective strengths of alloy solders respectively having different carbon contents were measured. It was found that the alloy solder having a carbon content of 0.7 wt % had a strength exceeding maximum strength required for soldering in various fields. The alloy solder having a carbon content above 0.7 wt % has a low electric conductivity, which will be a problem in the practical application of the alloy solder. When the carbon content is above 0.7 wt %, it is very difficult to disperse carbon uniformly and it is difficult to ensure a practically acceptable quality. The atomic weight of C is smaller than those of Sn and Ag. Therefore, even if the carbon content is in the range of 0.01 to 0.7 wt %, the number of carbon atoms is not necessarily small. Thus, the upper limit of the carbon content is 0.7 wt %.

The carbon content is properly determined on the basis of strength, hardness and electric conductivity required by the uses of the solder.

Preferably carbon added to the solder is graphite of a hexagonal system. When carbon is graphite, the carbon is soft and hence the carbon can be distributed in a practically satisfactorily uniform distribution in a high-temperature atmosphere of a temperature in the range of 800° C. to 1200° C. When carbon is diamond-type carbon of a cubic system, the carbon is very hard and hence the carbon cannot be distributed in a practically satisfactorily uniform distribution in a high-temperature atmosphere of a temperature in the range of 800° C. to 1200° C.

The present invention will be concretely described in connection with experiments. An alloy solder of the present invention containing Sn, 3.5 wt % Ag and 0.03 wt % C (Specimen A) and an alloy solder of the present invention containing Sn, 0.7 wt % Cu and 0.05 wt % C (Specimen C) were used. The added carbon was graphite. The designation “Sn, 3.5 wt % Ag and 0.03 wt % C (Specimen A) indicates an alloy solder obtained by adding C to a Pb-free solder containing 96 wt % or above Sn as a base element, 3.5 wt % Ag. The designation “Sn, 0.7 wt % Cu and 0.05 wt % C (Specimen C) indicates an alloy solder obtained by adding 0.05 wt % C to a Pb-free solder containing 99 wt % or above Sn as a base element and 0.7 wt % Cu.

Experiment 1

The specimens A and C were examined by biaxial x-ray diffractometry. Peaks presumably corresponding to carbon appeared in the results of examination for both the specimens A and C.

Experiment 2

The surfaces of the specimens were observed through a SEM (scanning electron microscope). The specimen A was compared with a specimen B of a solder containing Sn and 3.5 wt % Ag to which C was not added. Whereas any black substance presumed to be C was found in the specimen B, C was distributed substantially uniformly in the specimen A. The specimen C was compared with a specimen D of a solder containing Sn and 0.7 wt % Cu. Whereas any black substance presumed to be C was found in the specimen D, C was distributed substantially uniformly in the specimen C. Results of Experiment 2 proved that C having a melting point higher by far than the temperature of the high-temperature atmosphere in the range of 800° C. and 1200° C. used for producing the specimens A and C was not melted and was embedded in the specimens.

Experiment 3

The respective melting points of the specimens were measured by DSC (differential scanning calorimetry). Measured data is shown in Tables 1 to 4.

TABLE 1 1 ... Measured melting point of an alloy solder containing Sn, 3.5 wt % Ag and 0.03 wt % C.

TABLE 2 1 ... Measured melting point of an alloy solder containing Sn and 3.5 wt % Ag

TABLE 3 1 ... Measured melting point of an alloy solder containing Sn, 0.7 wt % Cu and 0.05 wt % C.

TABLE 4 1 ... 1 ... Measured melting point of an alloy solder containing Sn and 0.7 wt % Cu

It is known from Tables 1 and 2 that respective melting points of the specimens A and B are scarcely different from each other. It is known from Tables 3 and 4 that respective melting points of the specimens C and D are scarcely different from each other. Results of Experiment 3 prove that the addition of C to the alloy solder scarcely changes the melting point of the alloy solder, which is due to the unmelted C contained in the specimens A and C. Any changes do not occur in C at all at temperatures equal to the melting points of the specimens A and C. Therefore, the respective melting points of the specimens A and C are substantially equal to those of the specimens B and D, respectively.

Experiment 4

The respective resistivities (μΩcm) of a Pb—Sn alloy solder and the specimens A and B and the specimens C and D were measured. Electrical resistivity is an important factor of materials for connecting electronic parts. Measured data is shown in Table 5. In FIG. 5, a bar at the left position indicates the electrical resistivity of the conventional Pb—Sn alloy solder to which C was not added, bars at the middle position indicate the resistivities of the specimens A and B and bars at a right position indicate the resistivities of the specimens C and D.

TABLE 5

It is known from Table 5 that the respective resistivities of the specimens B and D of Pb-free alloy solders are the respective resistivities of the specimens A and C of the Pb-free alloy solders containing C is still lower. It is obvious from Table 5 that the effect of C on reducing electrical resistivity is conspicuous with the alloy solder containing Sn and 3.56 wt % Ag. It is considered that the reduction of resistivity and the augmentation of electric conductivity resulting from the addition of C are caused by the adsorption of intermetallic compounds by C having a large surface area to suppress the growth of intermetallic compounds and the subdivision of phases. Such augmentation of electric conductivity is a very desirable change in the property of the solder resulting from the addition of C to the solder.

Experiment 5

The respective Vickers hardnesses of the Pb—Sn alloy solder and the specimens A and B and the specimens C and D were measured. Measured data is shown in Table 6.

TABLE 6

As obvious from the comparison of the specimens A and B and the comparison of the specimens C and D, addition of C to the solder augments the hardness of the solder. The specimens A and C containing C have hardnesses higher than that of the Pb—Sn alloy solder. It is considered that the augmentation of hardness, similarly to the reduction of electrical resistivity, resulting from the addition of C is caused by the adsorption of intermetallic compounds by C having a large surface area to suppress the growth of intermetallic compounds and the subdivision of phases.

Experiment 6

It is known that the alloy solder and, for example, a Cu substrate are bonded together by alloying resulting from the diffusion of Cu from the Cu substrate into the molten alloy solder. A substance produced by alloying is called an intermetallic compound (IMC). The intermetallic compound is hard and brittle and has a low electric conductivity. Thus the intermetallic compound deteriorates the reliability of soldering. Although bonding of the alloy solder and the substrate cannot be achieved without producing an intermetallic compound, it is desirable that the layer of the intermetallic compound is as thin as possible and is firmly bonded to the substrate.

FIG. 1 is an image of the interface between a Cu substrate and the specimen A containing Sn, 3.5 wt % Ag and 0.03 wt % C and bonded to the Cu substrate formed by a SEM. FIG. 2 is an image of the interface between a Cu substrate and the specimen B containing Sn and 3.5 wt % Ag and bonded to the Cu substrate formed by a SEM. It is known through the observation of the image of the interface between the specimen A and the Cu substrate formed by a SEM that the interface is fine and has a uniform shape. It is known through the observation of the image of the interface between the specimen B and the Cu substrate formed by a SEM that the interface is coarse and considerably irregular. Since the surface area of the interface between the specimen A and the Cu substrate is larger than that of the interface between the specimen B and the Cu substrate, it is presumed that the specimen A is bonded to the Cu substrate more firmly than the specimen B.

It is known from the image of the interface between the specimen B and the Cu substrate formed by a SEM that cracks are formed in the interface. It is known from the image of the interface between the specimen A and the Cu substrate formed by a SEM that any cracks are not formed in the interface.

Weakening of a solder-bonded part resulting from the development of cracks in the interface between the conventional Pb-free solder, such as the specimen B, and the member to which the Pb-free solder bonds is one of serious drawbacks of the conventional Pb-free solder. In view of this fact, the crack-free interface between the specimen A and the Cu substrate as shown in FIG. 1 is a very significant advantage. It is considered that it is important that C contained in the intermetallic compound, such as Cu₆Sn₅, formed in the interface between the specimen A and the Cu substrate and not having cracks is graphite. It is inferred that graphite type carbon of a hexagonal system added to the alloy solder serves as a mechanical relaxation mechanism. In graphite type carbon, atoms are bonded by covalent bond of high bond strength in the surface and atoms are bonded by low van der Waals force with respect to the c-axis. It is inferred that the intermetallic compound can be formed between layers extending in the c-axis and a mechanical relaxation mechanism is formed. Thus, it is considered that addition of graphite type carbon not diamond type carbon of a cubic system to the alloy solder is an important factor in inhibiting the development of cracks. When diamond type of carbon of a cubic system is added to the alloy solder, atoms in a crystal structures are bonded by strong covalent bond and hence carbon can hardly serve as a mechanical relaxation mechanism.

The boundary between Cu and Sn that could not be clearly seen in the image formed by SEM could be clearly seen in an enlarged picture element mapping image, not shown, in the interface. Thus, the area of a part in which Cu and Sn are mixed can be measured and the area indicates an area in which the intermetallic compound is formed. The area of the intermetallic compound in the specimen A was larger than that in the specimen B. Thus, the effect of the added carbon on the improvement of bonding reliability could be ensured from the enlarged picture element mapping image.

Experiment 7

The Pb—Sn alloy solder and the specimens A, B, C and D were subjected to a tensile test for the measurement of yield stress and tensile strength. Measured data is shown in Tables 7 and 8

TABLE 7

TABLE 8

As obvious from Tables 7 and 8, the yield strength and tensile strength of the specimen A are higher than those of the specimen B and the yield strength and tensile strength of the specimen C are higher than those of the specimen D. Thus, it is known from the measured data that the addition of carbon is effective in enhancing yield strength and tensile strength. The tensile strength of the specimen A was higher than that of the Pb—Sn alloy solder.

As mentioned above, it is recognized that the addition of carbon to the Pb-free alloy solder is very effective in enhancing hardness and tensile strength and particularly in improving bonding reliability.

Pb-free solders to which carbon can be added include a Pb-free solder containing 96.5 wt % Sn, 3 wt % Ag and 0.5 wt % Cu, and a Pb-free solder containing 99.3 wt % Sn and 0.7 wt % Cu or 99.0 wt % Sn, 0.7 wt % Cu and 0.3 wt % Ag.

Processes of an alloy solder manufacturing method of manufacturing an alloy solder produced by adding carbon to a Pb-free solder in a preferred embodiment according to the present invention will be described in sequence. It is to be noted that the present invention is not limited to the alloy solder manufacturing method specifically described herein.

FIG. 3 is a schematic view of a high-temperature metal melting system 1. The high-temperature metal melting system 1 includes a melting furnace 2 to be charged with a Pb-free solder and powder or granular carbon, a heating space forming unit 3 for forming a sealed heating space over the melting furnace 2, a heating unit 4 including a plurality of gas burners that supply a heating fuel into and burn the heating fuel in the sealed heating space 6 to heat the sealed heating space 6 and the melting furnace 2, and an exhaust pipe 7 through which a gas produced in the sealed heating space 6 is exhausted. A mixture 5 of the Pb-free solder and a recarburizer is charged into the melting furnace 2 of the high-temperature metal melting system 1 and the mixture 5 is heated at a temperature in the range of 800° C. to 1200° C. by heating the sealed heating space 6 by the heating unit 4

FIG. 4 is a schematic view of a low-temperature metal melting system 8. The low-temperature metal melting system 8 includes a low-temperature melting furnace 9 to be charged with a solid mixture 11 solidified by a cooling process and a low-temperature heating unit 10 including a plurality of gas burners and disposed under the low-temperature furnace 9. The solid mixture 11 solidified by the cooling process is put in the low-temperature furnace 9 of the low-temperature metal melting system 8. The low-temperature heating unit 10 heats the solid mixture 11 at a temperature in the range of 250° C. to 400° C.

The alloy solder manufacturing method of the present invention includes a melting process for melting a Pb-free solder by heating the high-temperature metal melting system 1 charged with a Pb-free solder in a high-temperature atmosphere of a temperature in the range of 800° C. to 1200° C., a carburizing process for carburizing the Pb-free solder, namely, the molten Pb-free solder, held in the high-temperature atmosphere by adding powder or granular carbon as a recarburizer to the molten Pb-free solder such that a mixture of the Pb-free solder and carbon has a predetermined carbon content, a stirring process of stirring a mixture of the molten Pb-free solder and carbon, and a cooling process of cooling and solidifying the mixture of the Pb-free solder and carbon stirred in the stirring process and by pouring the mixture into a mold. The predetermined carbon content is in the range of 0.01 to 0.7 wt %.

In the alloy solder manufacturing method, the supply of the heating fuel is regulated such that a gas exhausted through the exhaust pipe 7 of the high-temperature metal melting system 1 by the melting process contains no oxygen. Thus, the oxidation of the mixture 5 of the solder and the recarburizer contained in the furnace 2 can be prevented.

Since the cooling process solidifies the mixture 5 by cooling the mixture 5 at a high temperature in the range of 800° C. to 1200° C., Sn and Ag are liable to separate and it is possible that Sn and Ag segregate. Therefore, the solidified mixture 5 is melted in a low-temperature atmosphere of a temperature in the range of 250° C. to 400° C. by the low-temperature metal melting system 8 to distribute segregated Sn and Ag uniformly in the mixture 5.

A carbon content adjusting process is executed after charging the low-temperature metal melting system 8 with the mixture 5. The carbon content adjusting process adjusts the carbon content of the mixture 5 to a desired carbon content by adding an additional amount of the Pb-free solder to the mixture 5. For example, about 100 kgw of an alloy solder having a carbon content of 0.05 wt % can be manufactured by adding an additional amount of 90 kgw of the Pb-free solder to 10 kgw of the mixture 5 having a carbon content of 0.5 wt %, namely, an alloy solder in the carbon content adjusting process executed by the low-temperature metal melting system 8. When the carbon content adjusting process executed by the low-temperature metal melting system 8 intends to manufacture an alloy solder having a carbon content of, for example, 0.05 wt % as a final product, the alloy solder can be very efficiently manufactured by manufacturing a mixture having a high carbon content in the range of 0.01 to 0.7 wt % by the high-temperature meal melting system 1 and executing the carbon content adjusting process by the low-temperature metal melting system 8 as compared with efficiency at which the alloy solder can be manufactured only by the high-temperature metal melting system 1. The manufacturing cost of manufacturing the final product can be reduced.

The mixture 11 having the desired carbon content determined by the carbon content adjusting process is poured into a mold again and the mixture 11 is cooled and solidified by a recooling process. Since the recooling process is executed in a low-temperature atmosphere of a temperature in the range of 250° C. to 400° C., the segregated elements can be desegregated.

An analyzing process for analyzing the solidified mixture 5 solidified by the cooling process to determine the carbon content may be executed prior to the carbon content adjusting process. Thus, the carbon content adjusting process can accurately determine the additional amount on the basis of the results of analysis made in the analyzing process.

EXAMPLES

First, a Pb-free solder containing 96.5 wt % Sn, 3 wt % Ag and 0.5 wt % Cu and generally called a 305 series alloy is charged into the high-temperature metal melting system 1 and the high-temperature metal melting system 1 is heated to create a high-temperature atmosphere of 100° C. to melt the 305 series alloy (melting process).

The heating fuel supply rate is regulated so that a gas exhausted through the exhaust pipe of the metal melting furnace dos not contain any oxygen and the heating fuel may burn completely. Otherwise, carbon burns and carburizing efficiency drops.

Then, a recarburizer for carburizing iron is added to the 305 series alloy melted by the melting process, namely, the molten 305 series alloy, and contained in the high-temperature metal melting system 1 such that the molten 305 alloy has a carbon content of 0.5 wt % (carburizing process).

A mixture of the molten 305 series alloy and the recarburizer contained in the high-temperature metal melting system 1 is stirred to mix the molten 305 series alloy and the recarburizer uniformly (stirring process).

The mixture of the molten 305 series alloy and the recarburizer stirred by the stirring process is poured into a mold to cool and solidify the mixture (cooling process).

The mixture of the molten 305 series alloy and the recarburizer solidified by the cooling process is analyzed to determine the carbon content of the mixture (analyzing process).

The mixture 5 is charged into the low-temperature metal melting system 8 to melt the mixture 5 and an amount of the 305 series alloy determined on the basis of the carbon content determined by the analyzing process is added to the molten mixture 5 to adjust the carbon content of the molten mixture 5 to 0.1 wt % (carbon content adjusting process).

Lastly, the mixture 11 having a desired carbon content adjusted by the carbon content adjusting process is poured into a mold again to cool and solidify the mixture 11 (recooling process).

A problem of the bonding strength of a Pb-free alloy solder being inferior to that of the conventional Pb-containing alloy is solved and an alloy solder having bonding reliability exceeding that of the Pb-free solder can be manufactured by carburizing the Pb-free solder by the foregoing processes.

Although this embodiment uses the 305 series alloy as the Pb-free solder, the Pb-free solder is not limited thereto and may be, for example, a Pb-free solder containing 99.3 wt % Sn and 0.7 wt % Cu or a Pb-free solder containing 99.0 wt % Sn, 0.7 wt % Cu and 0.3 wt % Ag.

Although this embodiment uses the high-temperature atmosphere of 1000° C., the temperature of the high-temperature atmosphere is not limited thereto and may be any one of temperatures in the range of 800° C. to 1200° C.

Although the carbon content of this embodiment described herein is 0.5 wt %, the carbon content is not limited thereto and may be any one of carbon contents in the range of 0.01 to 0.7 wt %.

Although the carbon content is adjusted to 0.1 wt % by the carbon content adjusting process in this embodiment, the carbon content is not limited thereto and may be adjusted to a proper carbon content according to the use of the solder alloy.

INDUSTRIAL APPLICABILITY

The technical idea of the present invention is properly applicable to fields to which the technical idea of the present invention is applicable. The present invention is applicable particularly to the field of vehicles in which bonding reliability is important. Thus, the present invention is applicable to a variety of industrial fields. 

1-18. (canceled)
 19. An alloy solder manufactured by adding carbon to a Pb-free solder in a predetermined carbon content in a range of 0.01 to 0.7 wt %, the carbon being graphite of a hexagonal system.
 20. The alloy solder according to claim 19, wherein the carbon is added in a high-temperature atmosphere in a range of 800° C. to 1200° C.
 21. The alloy solder according to claim 19, wherein the Pb-free solder contains 96.5 wt % Sn, 3 wt % Ag and 0.5 wt % Cu.
 22. The alloy solder according to claim 19, wherein the Pb-free solder contains 99.3 Wt % Sn and 0.7 wt % Cu.
 23. The alloy solder according to claim 19, wherein the Pb-free solder contains 99.0 wt % Sn, 0.7 wt % Cu and 0.3 wt % Ag.
 24. An alloy solder manufacturing method comprising: a melting process for melting a Pb-free solder by heating a high-temperature metal melting system containing the Pb-free solder at a temperature for a high-temperature atmosphere in a range of 800° C. to 1200° C.; a carburizing process for carburizing the molten Pb-free solder held in the high-temperature atmosphere by adding carbon as a recarburizer to the molten Pb-free solder such that the Pb-free solder has a predetermined carbon content in a range of 0.01 to 0.7 wt %; a stirring process for stirring a mixture of the molten Pb-free solder and carbon; and a cooling process for cooling the mixture of the Pb-free solder and carbon stirred by the stirring process for stirring the mixture of the Pb-free solder and carbon and poured into a mold to solidify the mixture.
 25. The alloy solder manufacturing method according to claim 24, wherein the high-temperature metal melting system includes a melting furnace to be charged with the Pb-free solder and the recarburizer, a sealed heating space forming part for forming a sealed heating space over the melting furnace, a heating unit for supplying a heating fuel into the sealed heating space to heat the sealed heating space and the furnace, and an exhaust pipe connected to the heating space forming part.
 26. The alloy solder manufacturing method according to claim 25, wherein the supply of the heating fuel is regulated such that a gas exhausted through the exhaust pipe during the melting process contains no oxygen.
 27. The alloy solder manufacturing method according to claim 24 further comprising: a carbon content adjusting process for adjusting the carbon content of the mixture to a desired carbon content by charging a low-temperature melting system with the mixture solidified by the cooling process, melting the mixture in a low-temperature atmosphere and charging a predetermined additional amount of the Pb-free solder into the low-temperature melting system such that the mixture has a desired carbon content; and a recooling process for cooling and solidifying the mixture having the desired carbon content adjusted by the carbon content adjusting process by pouring the molten mixture into a mold.
 28. The alloy solder manufacturing method according to claim 27 further comprising an analyzing process for determining the carbon content of the mixture solidified by the cooling process to be executed before the carbon content adjusting process; wherein the carbon content adjusting process determines the predetermined additional amount on the basis of the result of the analysis made by the analyzing process.
 29. The alloy solder manufacturing method according to claim 27, wherein the mixture solidified by the cooling process has a higher one of carbon contents in the range of 0.01 to 0.7 wt %, and the desired carbon content is a lower one of the carbon contents in the range of 0.01 to 0.7 wt %.
 30. The alloy solder manufacturing method according to claim 27, wherein the temperature of the low-temperature atmosphere is in the range of 250° C. to 400° C.
 31. The alloy solder manufacturing method according to claim 24, wherein the carbon is graphite of a hexagonal system.
 32. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 24. 33. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 25. 34. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 26. 35. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 27. 36. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 28. 37. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 29. 38. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 30. 39. An alloy solder manufactured by the alloy solder manufacturing method according to claim
 31. 